Didcot power station cooling towers

The causes of global warming

The world is warming, but what are the causes?

Natural causes?

Over time, and long before there were people to cause it, the world’s temperature has varied greatly. So could natural causes be the reason for the current warming?

It turns out that natural causes are tending to reduce temperatures, not raise them.

Solar radiation

As can be seen from the graph, the amount of solar radiation is currently reducing at the same time as the earth’s temperature is rising. This isn’t the cause of global warming.

Rising temperatures (upper graph) at the same time that solar radiation has been falling (lower graph).

Volcanoes

You might expect volcanic eruptions would increase atmospheric temperatures because of the hot ash and gases they release. But it turns out that volcanoes tend to cool the earth because the ash in the air reflects the incoming sunlight.

Human causes

The current temperature rise of more than 1 degree Celsius has occurred since the industrial revolution (about 1800) and is almost totally due to human activity increasing the amounts of Greenhouse gases in the atmosphere.

Global temperatures over the past 1700 years

Greenhouse gases such as carbon dioxide, nitrous oxide and methane trap some of the sun’s heat and prevent it reflecting back into space. These gases have been increasing since the industrial revolution.

Greenhouse gas concentrations for 2000 years (American Chemical Society, from IPCC Fourth Assessment Report (2007)). Recent values were measured directly, historic values were obtained from ice cores.

Sources of greenhouse gases

The main contributors to this increase in Greenhouse gases have been:

  • burning coal, oil and gas (about 70%),
  • agricultural activities and deforestation (about 20%), and
  • industrial and waste processes (about 10%).

It is clear that most of these sources have been increasing since the industrial revolution and will continue to increase while fossil fuels (oil, coal and gas) use is increasing.

This is why ending fossil fuel use as quickly as possible is critical for combatting climate change.

Further Reading

Photo: Didcot gas and coal fired power station in UK, now demolished (Morguefile).